A superelastic alloy has a much wider elastic range than other metal materials at a temperature equal to or higher than the reverse transformation temperature, and immediately recovers an original shape even when deformed. The superelastic alloy is an alloy material which is expected to be applied to medical fields and medical devices such as orthodontics tools, catheters, stents, bone plates, coils, guide wires and clips by virtue of the above-mentioned property.
Studies on superelastic alloys are conducted with various kinds of alloy systems on the basis of knowledge about shape memory alloys. Superelastic alloys that are currently most well known in terms of practical use include Ni—Ti-based shape memory alloys. Ni—Ti-based shape memory alloys have a reverse transformation temperature of 100° C. or lower, can exhibit superelasticity at the body temperature of a human being, and therefore may be qualitatively applicable to medical devices. However, Ni—Ti-based shape memory alloys contain Ni for which there is a concern about biocompatibility which is associated with metal allergy. Biocompatibility is a matter that may be fetal when application to medical fields is considered.
Thus, alloy materials which can exhibit superelastic property while being Ni-free have been developed. For example, Patent Document 1 discloses a Ti alloy obtained by adding Mo and one of Al, Ga and Ge to Ti. In the Ti alloy, Mo is added as an additive element having an action of stabilizing the β phase of Ti, and Al, Ga or Ge that acts to stabilize the α phase of Ti is added while consideration is given to the biocompatibility of each additive element, and the contents of these additive elements are optimized to exhibit superelastic property. Additionally, it has been reported that various kinds of Ti-based alloys such as Ti—Nb—Al alloys and Ti—Nb—Sn alloys can exhibit superelastic property.